This invention relates to improved methods and apparatus for depositing thin films of hydrogenated amorphous silicon semiconductor materials. Films of hydrogenated amorphous silicon have particular utility for semiconductor applications known in the art such as photovoltaic solar cells, optical sensors, electrophotography, thin film transistors, luminescent displays, optical data recording media and the like. the properties of amorphous silicon semiconductor thin films have been found to depend critically on deposition conditions such as growth rate, chemical precursor species distribution, impurities and temperature of growth. Reproducibly achieving the desired properties has been the object of much research effort.
Deposition of amorphous silicon films is conventionally accomplished by glow discharge decomposition of silane such as described in U.S. Pat. No. 4,064,521 issued Dec. 20, 1977 to D. E. Carlson. Glow discharge processes for depositing amorphous silicon semiconductor thin films utilize plasmas for generating silicon containing radicals which are precursors to film growth. By careful control of gas composition, flow rate, pressure, plasma power, bias and substrate temperature, useful hydrogenated amorphous silicon semiconductor thin films have been obtained. However, glow discharge deposition of large areas is characterized by low film deposition rates, poor utilization of raw materials and the costly plasma generating and control equipment.
An alternative deposition technique is prolytic decomposition of disilane, commonly known as chemical vapor deposition. Chemical vapor deposition of hydrogenated amorphous silicon is described in an article by S. C. Gau et al entitled "Preparation of Amorphous Silicon Films by Chemical Vapor Deposition from Higher Silanes" which was published in Applied Physics Letters, Volume 39, No. 1, pp. 436-38, Sept. 1, 1981. In the chemical vapor deposition method, a substrate is heated to a temperature of at least 380.degree. C. in the presence of disilane gas. However, amorphous silicon films grown at such temperatures have proven to be unsuitable.
Useful amosphous silicon semiconductor thin films have been deposited by thermal generation of the precursors to film growth. Earlier work of applicant is described in U.S. Pat. Nos. 4,237,150 and 4,237,151, the details of which are incorporated herein by reference thereto. These patents disclose methods for producing hydrogenated amorphous silicon by thermally decomposing a silicon bearing gas or gases.
U.S. Pat. No. 4,237,151 to Strongin et al, issued Dec. 2, 1980, describes feeding a silicon bearing gas into a tube heated to temperatures above 1700.degree. C. wherein the gas thermally decomposes forming elemental silicon and hydrogen. The tube wall is provided with an opening, through which a flux of decomposition products effuses into vacuum less than 10.sup.-4 torr. The flux of hydrogen and silicon condense on a substrate to form film. A major disadvantage with the method of U.S. Pat. No. 4,237,151 is that the tube and opening radily become clogged with solid products of the decomposition reaction. Furthermore, when the temperature of the tube is raised or when the tube is placed closer to the substrate for the purpose of increasing the deposition rate, excess heating of the substrate occurs.
In applicant's U.S. Pat. No. 4,237,150 a stream of silicon and hydrogen bearing gas or gases, at a pressure of approximately 10.sup.-4 torr, is directed against a heated surface being approximately 1400.degree. C. The silicon bearing gas decomposes on the heated surface and the resulting decomposition products are condensed onto a suitable substrate which is heated to a temperature of 600.degree. C. or less. Condensing the thermal decomposition products of the gaseous mixture onto a substrate heated to approximately 225.degree. C. resulted in films of hydrogenated amorphous silicon with low dark conductivity, high photoconductivity and dark conductivity activation energy of 0.7 eV. A problem with the methods disclosed in U.S. Pat. No. 4,237,150 is excessive heating of the substrate which occurs when the heated surface is put in close proximity to the substrate and the pressure of the gas are raised for the purpose of accelerating the deposition rate of the deposited film.
B. A. Scott et al, in an article entitled "Kinetics and Mechanism of Amorphous Silicon Growth by Homogeneous Chemical Vapor Deposition", published in Applied Physics Letters, Volume 39, pages 73-75 (1981), July 1, 1981, describe a technique wherein a thin film of hydrogenated amorphous silicon is grown on a substrate held at temperatures less than 400.degree. C. In the technique of Scott et al, monosilane is pumped through a furnace heated reactor containing a pedestal with a nitrogen cooled metal block substrate holder. The walls of the reactor are heated to a temperature of 550.degree. C. or higher. Silane gas, flowing through the tube at a pressure of 5 torr is decomposed in the region of the heated walls to produce reactive free radial intermediates which are believed to be silyene. The free radicals diffuse to the substrate wherein they react yielding the thin film of hydrogenated amorphous silicon. In order to maintain the desired substrate temperature for growing the film, the substrate was clamped to the metal block holder and the flow rate of nitrogen coolant adjusted.
A feature common to the methods of Scott et al and U.S. Pat. No. 4,237,150 is that they employ apparatus in which the area heated above the decomposition temperatures of the gas is equal to or greater than the area of the substrate. As a result, the heat load on the substrate is excessive and control of the substrate temperature requires elaborate cooling schemes which preclude low cost, high throughput production of large areas of substrate coated with amorphous silicon films having the desired properties.